Diagnostic Testing In COVID-19


A detailed clinical history regarding the onset and duration of symptoms, travel history, exposure to people with COVID-19 infection, underlying preexisting medical conditions, and drug history should be elicited by treating providers.

Patients with typical clinical signs suspicious of COVID-19 such as fever, cough, sore throat, loss of taste or smell, malaise, and myalgias should be promptly tested for SARS-CoV-2. Besides symptomatic patients, patients with atypical symptoms of COVID-19 or anyone with known high-risk exposure to SARS-CoV-2 should be tested for SARS-CoV-2 infection even in the absence of symptoms.

Diagnostic Testing In COVID-19

Molecular Testing

  • The standard diagnostic mode of testing is testing a nasopharyngeal swab for SARS-CoV-2 nucleic acid using a real-time PCR assay. Commercial PCR assays have been validated by the US Food and Drug Administration (FDA) with emergency use authorizations (EUAs) for the qualitative detection of nucleic acid from SARS-CoV-2 from specimens obtained from nasopharyngeal swabs as well as other sites such as oropharyngeal, anterior/mid-turbinate nasal swabs, nasopharyngeal aspirates, bronchoalveolar lavage (BAL) and saliva. The collection of BAL samples should only be performed in mechanically ventilated patients as lower respiratory tract samples seem to remain positive for a more extended period.
  • The sensitivity of PCR testing is dependent on multiple factors that include the adequacy of the specimen, technical specimen collection, time from exposure, and specimen source.[94] However, the specificity of most commercial FDA-approved SARS-CoV-2 PCR assays is nearly 100%, provided that there is no cross-contamination during specimen processing.
  • SARS-CoV-2 antigen tests are less sensitive but have a faster turnaround time compared to molecular PCR testing.[95] Comprehensive testing for other respiratory viral pathogens should be considered for appropriate patients as well.

Serology Testing

  • An antibody test can evaluate for the presence of antibodies that occurs as a result of infection. Antibody tests play an important role in broad-based surveillance of COVID-19, and many commercial manufactured antibody testing kits are available to evaluate the presence of antibodies against SARS-CoV-2 are available.
  • Despite the numerous antibody tests designed to date, serologic testing has limitations in specificity and sensitivity, and results from different tests vary. However, an antibody test with a specificity higher than 99% and a sensitivity of 96% has been developed by the CDC, which can identify past SARS-CoV-2 infection.
  • Antibody testing may be instrumental in broad-based surveillance of COVID-19 and evaluate the immunity conferred from infection or vaccination. There is currently ongoing research to determine quantitative and qualitative aspects of antibodies regarding protection from future SARS-CoV-2 infection and the duration of the protection.

Other Laboratory Assessment

Imaging Modalities

Considering this viral illness commonly manifests itself as pneumonia, radiological imaging has a fundamental role in the diagnostic process, management, and follow-up. Imaging studies may include chest x-ray, lung ultrasound, or chest computed tomography (CT). There are no guidelines available regarding the timing and choice of pulmonary imaging studies in patients with COVID-19, and the type of imaging should be considered based on clinical evaluation.

Chest X-ray 

  • Standard radiographic examination (X-ray) of the chest has a low sensitivity in identifying early lung changes; it can be completely normal in the initial stages of the disease.
  • In the more advanced stages of infection, the chest X-ray examination commonly shows bilateral multifocal alveolar opacities, which tend to confluence up to the complete opacity of the lung. Pleural effusion can also be demonstrated.

Chest Computed Tomography (CT)

  • The American College of Radiology recommends against Chest CT’s routine use as an initial imaging study or screening.
  • Given its high sensitivity, chest computed tomography (CT), particularly high-resolution CT (HRCT), is the diagnostic method of choice in evaluating COVID-19 pneumonia, particularly when associated with disease progression.
  • Several non-specific findings and radiologic patterns can be found on Chest CT. Most of these findings may also be observed in other lung infections, such as Influenza A (H1N1), CMV, SARS, MERS, streptococcus, and Chlamydia, Mycoplasma.
  • The most common CT findings in COVID-19 are multifocal bilateral “ground or ground glass” (GG) areas associated with consolidation areas with patchy distribution, mainly peripheral/subpleural, and greater involvement of the posterior regions lower lobes. The “crazy paving” pattern can also be observed.
  • This latter finding is characterized by GG areas with superimposed interlobular septal thickening and intralobular septal thickening. It is a non-specific finding that can be detected in different conditions.
  • Other notable findings include the “reversed halo sign,” a focal area of GG delimited by a peripheral ring with consolidation, and the findings of cavitations, calcifications, lymphadenopathies, and pleural effusion. 

Lung Ultrasound

Ultrasonographic examination of the lung allows evaluating the progression of the disease, from a focal interstitial pattern up to a “white lung” with evidence of subpleural consolidations. Considering its noninvasive nature and zero risks of radiation, it is a useful diagnostic modality for patient follow-up and assists in determining the setting of mechanical ventilation and prone positioning. The main sonographic features are:

  • Pleural lines: appear often thickened, irregular, and discontinuous until it almost seems erratic; subpleural lesions can be seen as small patchy consolidations or nodules.
  • B lines: They are often motionless, coalescent, and cascade and can flow up to the square of “white lung.”
  • Thickenings: They are most evident in the posterior and bilateral fields, especially in the lower fields; the dynamic air bronchogram within the consolidation is a manifestation of disease evolution.
  • Perilesional pleural effusion

In summary, during the course of the illness, it is possible to identify the first phase with focal areas of fixed B lines followed by a phase of numerical increase of the lines B up to the white lung with small subpleural thickening, which progresses further until there is evidence of posterior consolidations.

reference link: https://www.ncbi.nlm.nih.gov/books/NBK554776/#article-52171.s9

Controversy of chest CT versus RT-PCR

Patients with SARS-Cov-2 infection can experience a diverse range of clinical presentations, from no symptoms to acute respiratory distress syndrome (ARDS), septic shock, and/or multiple organ failure [4, 5]. SARS-Cov-2 virus is detectable in the respiratory tract 2–3 days before symptom onset, peaks at symptom onset, and declines over the following 7–8 days [6]. Difficulties in infection control of COVID-19 are in part ascribable to this viral shedding profile, contrasting to that of influenza virus that peaks after symptom onset [7].

RT-PCR is currently the most reliable diagnostic tool for COVID-19 [8]. Specimens obtained from a nasopharyngeal or oropharyngeal swab are commonly used [7]. However, the false-negative rate of RT-PCR test is not negligible, estimated as 100% on the day of infection (day1) and 38% on the day of symptom onset (day 5), which decreases to 20% at 3 days after symptom onset (day 8) and increases again thereafter [9].

The instability of RT-PCR may be ascribed to variabilities in viral load depending on the disease stage and sampling error [5, 7, 8]. Bronchoalveolar lavage is more sensitive than RT-PCR but not realistic for route application [10]. In addition, in the early phase of the pandemic, the use of RT-PCR was limited because of logistical issues—including the development, mass production, and dissemination of the examination kit. The turnaround time of RT-PCR was several days in early 2020 [11].

Given these limitations of RT-PCR, the use of chest CT was widely debated since the early period of the pandemic, especially in the context of replacing RT-PCR as a diagnostic tool [12, 13]. Chest CT was deemed more available in many hospitals and possibly able to achieve a superior diagnostic performance in the early period of infection [14]. Early radiological studies from China spurred this discussion. On February 2020, a study from China first reported that 5 symptomatic patients showed chest CT abnormalities despite initial PCR negative results [15].

A subsequent study from China reported that chest CT showed a higher sensitivity than RT-PCR (98% vs. 71%) [16]. Similar findings were also reported from a larger cohort study from China that investigated 1014 patients and revealed the sensitivity of chest CT and RT-PCR to be 97% and 88%, respectively, based on which they recommended chest CT for screening of COVID-19 instead of RT-PCR [17]. Finally, a meta-analysis from Korea summarizing early reports published within the first 1 month of the pandemic reported that the sensitivity of chest CT exceeds that of RT-PCR (93% vs. 89%) with a specificity of 37% [18].

However, problems were noted in many of the early studies published within 1 months of the pandemic: (1) Patient background was not specified; (2) disease severity of the cohort was biased toward severe and hospitalized cases; (3) the indications for performing chest CT scan were not described; (4) a definition of positive chest CT findings was not provided (despite positive CT findings were not equal to positive CT findings of COVID-19); (5) nonuniformity of the reference standard [19,20,22]. Antithesis of screening by chest CT was presented in March 2020 by a Japanese study of a mass infection cohort, reporting the sensitivity of chest CT to be 79% in symptomatic and 54% in asymptomatic patients [23, 24].

Based on a risk–benefit analysis including diagnostic performance, medical cost, precaution issue, and risk of radiation exposure, medical specialty societies published position statements against the use of chest CT for screening of COVID-19 including the American College of Radiology (ACR), the Society of Thoracic Radiology (STR), and the American Society of Emergency Radiology (ASER) in March to April, 2020 [12, 25, 26]. The Fleischner Society also published a consensus statement of a multidisciplinary expert panel [27].

Their statement offered guidance for the use of chest imaging modalities in different healthcare environments and scenarios [27]. The multidisciplinary panel concluded that chest CT is not recommended for asymptomatic or mild symptomatic patients with COVID-19 in the absence of accompanying risk factors or routine screening in a resource abundant environment [27]. On the other hand, they recommended chest CT for medical triage of patients with suspected COVID-19, who present with moderate to severe clinical features and a high-pretest probability of disease in a resource-constrained environment [27].

They also recommended chest CT for patients with moderate to severe symptoms with suspected COVID-19 or those experiencing respiratory functional impairment, hypoxemia, or both after recovery from infection [27]. For these patients, imaging provides a baseline for future comparison, may reveal an alternative diagnosis, may establish manifestations of important comorbidities in patients with risk factors for disease progression, and may influence treatment strategy and the intensity of monitoring for clinical worsening [27].

They discouraged the use of chest CT for diagnostic purpose of COVID-19; however, situations are different in a resource-constrained environment, in which availability of RT-PCR is limited or at emergency room, where patients are critical condition necessitating prompt triage or unconscious and unable to speak their symptoms or exposure history [27].

Role of chest radiograph

Chest X-ray used to be deemed less useful than chest CT because of their lower sensitivity in the diagnosis of subtle parenchymal abnormalities and limited ability to help differentiate parenchymal patterns [28]. Chest CT is gold standard imaging technique for thoracic evaluation of COVID-19, but is not always available, for example, for unstable patients in the intensive care unit (ICU) with hypoxemia and hemodynamic failure [29].

For these patients, bedside chest X-ray is still the standard of care. Other advantages of chest X-ray include its ready and wide availability, making it possible to use in almost all clinical settings [28]. Chest X-ray is less-resource intensive, is achieved with lower radiation doses, is easier to repeat, and can be performed with portable equipment at the point of care, minimizing the risk of cross-infection related to patient transport [30]. Some early studies argued against the use of chest X-ray as the first-line imaging modality because of its low sensitivity in detecting alterations [31, 32]. In contrast, the later statements of several radiological societies have encouraged its use in combination with RT-PCR instead of CT [27, 33].

The sensitivity of chest X-ray depends mainly on two factors, i.e., symptom severity and disease stage [33, 34]. In relation to the former, Kuo et al. conducted a research to evaluate the screening value of chest X-ray with 1964 patients with COVID-19 who were asymptomatic or had mild symptoms as defined by the consensus statement of the Fleischner Society [35]. They demonstrated that only 39 patients (2.0%) showed abnormal findings on chest X-ray and full recovered after supplemental oxygenation and inpatient treatment [35].

The results of this study validated the Fleischner Society’s proposal for the first clinical scenario, i.e., chest imaging is not recommended for asymptomatic or mildly symptomatic patients. Although the amount of research focused on the other two clinical scenarios, i.e., mild to severe patients with abundant or limited-resources, some research may provide hints to them [4, 33, 36, 36]. One is an early study by Chen et al., in which their first 99 cases of COVID-19 in Wuhan, China, were described [4]. The study cohort seemingly comprised moderate to severe hospitalized patients, 33% of whom had organ dysfunction, and 100% showing chest radiograph abnormality on admission [4].

Several other studies investigated a less severe spectrum of patients [36, 37]. Wong et al. investigated a cohort of 64 patients (86% symptomatic and 14% asymptomatic), 69% of whom showed chest radiograph abnormalities on admission [36]. Toussie et al. investigated a cohort of 338 patients (43% inpatients and 57% outpatients), 50% of whom showed abnormalities on the first chest radiograph at a mean of 4 days after onset [37]. The sensitivity of chest X-ray as a function of disease course was investigated by Vancheri et al., who recruited 240 mildly symptomatic patients with COVID-19 [33]. They showed that the sensitivity of chest X-ray was 63.3% on day 0–2, 72% on day 3–5, 81.2% on day 6–9, and 83.9% on day > 9 [33].

In summary, for the general population, chest X-ray is not recommended as the first-line imaging modality for early disease or asymptomatic or mildly symptomatic patients because of limited sensitivity compared to CT [35, 38]. In contrast, for those with progressed or moderate to severe disease, chest X-ray may be an effective alternative for assessing disease progression; the need for chest CT may be negated with positive chest X-ray findings [39]. For patients sensitive to radiation exposure, i.e., pregnant women or pediatric patients, or unstable patients unable to be transported to the CT room, chest X-ray is a useful alternative method of chest CT [29, 40].

Role of ultrasound

With experience lung ultrasonography can be as useful as chest CT and superior to standard chest X-ray for evaluation of pneumonia and/or adult respiratory distress syndrome [40]. It has moreover the added benefits of ease of use, repeatability, no radiation exposure, and being cheap [40]. Point-of-care ultrasound using a hand-held mobile device enables assessments in various settings not only in emergency department and intensive care unit, but rural healthcare facilities, nursing homes, and aeromedical transport as well [41]. The appropriate use of ultrasonography could decrease chest X-ray and CT use in patients in the ICU [40].

Ultrasonography artefacts arising from the chest wall and pleural surfaces can provide valuable information about diverse lung pathologies either correlating or not correlating with the existing lung pathology of COVID-19 [40]. The normal lung back reflects ultrasound waves providing a transverse parallel hyperechoic lines called A-line [38]. With disease progression, new signs including pleural line (A-line) thickening and irregularities and B-line artifact, vertical hyperechoic lines starting from the pleura and continuing to the bottom of the image, may be noted [38, 42].

The presence of B-line will vary among focal, multifocal, and confluent patterns of involvement [40]. As B-lines reflect interstitial thickening and inflammation, the number increases with disease severity [42]. Consolidation and increased echogenicity of lung parenchyma with air-filled bronchi may also become apparent and increase in frequency and size [42].

The extent of consolidation may also vary becoming more prominent with the assumption of diverse patterns, from multifocal, small, subpleural consolidations to non-translobar and translobar involvement, and in some cases accompanied by air bronchograms [40]. The most specific finding of pneumonia is “Shred Sign”, which reflects an irregular shredded appearance at the interface between aerated normal and consolidated lung [38]. Pleural effusions are uncommon, with generally only those patients who are more critically ill showing them [40]. The presence of A-lines through the recovery phase is considered an indirect sign of recovery [40].

A meta-analysis by Barssoum et al. showed a sensitivity of lung ultrasound of 68–93.3% and of NPV of 52–94.1%, highlighting the value of lung ultrasound as a screening test to rule out COVID-19 pneumonia [43,44,45,46,47]. In contrast, the available data regarding specificity and PPV are conflicting with one study showing 92.9% and 84.6% for sensitivity and PPV, respectively, and another lower values of 21.3% and 19.2%, respectively [44, 47].

Lu et al. investigated the diagnostic performance of lung ultrasound with chest CT as the reference demonstrating high sensitivity and specificity in mild, moderate, and severe lung lesions with 68.8%, 77.8%, and 100.0% and 85.7%, 76.2%, and 92.9%, respectively [44].

figure 1
Example of chest CT patterns of RSNA classification. Axial CT images are categorized as (a) “Negative for pneumonia” meaning no features of pneumonia, (b) “Atypical appearance”, meaning typical for other infection but not COVID-19, e.g., bronchial pneumonia, lobar pneumonia, tuberculosis, or fungal infection, (c) “Indeterminate appearance”, meaning the presence of feature suspicious for COVID-19 but with overlaps with other diseases, drug-induced pneumonia, collagen disease-related lung diseases, or alveolar pulmonary edema, and (d) “Typical appearance”, meaning the typical pattern of COVID-19 pneumonia [49]

reference link : https://insightsimaging.springeropen.com/articles/10.1186/s13244-021-01096-1



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